Method of commutating alternating-current electric motors.



N0- 828,077. PATENTED AUG. '7, 1906.

' H. WEICHSEL. METHOD OF COMMUTATING ALTERNATING CURRENT ELECTRICMOTORS.

APPLICATION FILED AUG. 28, 1905.

-n'rrnn srArns P TENT orricn HANS WlDICHSEL, or WILKINSBURG', PENsrLvAnrA, METHOD- OF commumme ALTERNATING-VCURRENT ELECTRIC MOTORS.

fSpecification of Letters Patent.

Patented Aug. 7, 1906.

, Implication filed August 28, 1905 Serial No..276.015.

rent ElectricMotors, of which the-following is a specification,reference being had therein to the accompanying drawings.

This invention relates to a method of commutating alternating-currentelectric motors; and its object is to prevent, or, at least,greatlyreduce, the sparks at the commuta tor-brushes. This'result issecured by maintaining the short circuit of the armaturecoils forone-half o1 the period of the current, or approximately so.

In the accompanying drawings, Figure 1' is a diagrammatic view of themotor-windmgs and commutator, and Flg. 2 1s a diagramshowing thecharacteristlcs of the cur- V rent in my method of commutatin The motorwill be of any type 0 alternat ing current commutator motor-+that is,either a series-wound motor or an induction commutator-motor. The motorshown in Fig. 1 has-the field-coils Sand 6 and armature-coils, which arerepresented as an ordinary Gramme ring 7, the individual coils of whichare connected to the commutator-bars 8. The Gommutatonbrushes 9 and10b'ear u on these bars in the usual manner, and one t ereot isconnected; to the one field-coil 5 and'the other to the other field-coil6. One of the line-wires 11 is connected to the field- I coil 5, and theother line-wire 12 is connected to the field-coil 6.. There is nothingnew in this arrangement of motor, and 1t has been selected merely forpurpose of illustration;

In all alternating-current motors as at present constructed and operateda considerable amount of sparkin takes place at the These sparks are dueto a difference in the strength'or direction of the current flowingthrough a given armature-coil at the time of breaking the short circuitthereof and the current which must flow through that coil immediatelyafter the short circuit is broken. As is ,well known, the commutation iseffected by the brushes bridging adjacent commutator-bars, thus Short--ircuiting a coil of the armature for the period of time during whichthe brush bridges these bars and breaking said short-circuit when thebrush passes off one of said bars' Before the commutation begins thecurrent through the armature-coil is in the same direction as theline-current, but after the commutationthat is, after the short circuitis broken-the current through the said coil is opposite to theline-current.

- lhe sparks are due to the fact that the period of commutationthat is,the period of maintaining the short circuit of the coil-is of such briefduration that it is broken at a time when the current flowing throughthe A coil is op osite to that which must flow through t at coilimmediately thereafter. This may be explained by the diagram Fi 2. Insaid figure we will suppose the line a to represent the line-current, 13being the time period thereof and t one-half of said period.

which must flow through the observed arma- The dotted line 1) representsthe current turercoil immediately after the breaking of I the shortcircuitthat is, the commutated linecurrentthis current being opposite tothe external line-current a. If now we suppose the period ofcommutation-that is, the period of short-circuiting the observed coilegins at the maximum'of the linecurrentnamely, at l when this occurs,the line-current no lon er flows through the short-air cuited coil, utthrough the adjacent coil.

During this short-circuiting, however, current will flow through theshort-circuited coil, this current having two factors. One of saidfactors. is the residuum of the line-cur rent. If the coil hadabsolutely no reactance, the current through it would drop to zeroimmediately upon the making of the short circuit; but as all coilsnecessarily have reacance the current will die down gradually, the rateof drop depending upon the ratio of the resistance of the coil to itsreactance. This current may be represented by the line 0. The otherfactor of the current flowing through the short-circuited coil is theinduced current produced by the position of the close-cir'cuited coil inthe magnetic field and the rise and fall of said field. This induced Ifnow We IIO rent will be. at f. The current which must,

, flow througihnthe observed coil immediately rent which must flow nameacteristics of after b'rea g the short circuit must be equal to but inthe opposite direction to the line-current-that is, it will be negativeand at 9; but. the current e, flowing through the observed coil at thetime of breaking the short circuit, is at 'h and is positive. Thedifference between the resultant current at h and the current which mustflow through the coil after the breaking at 9 will cause a large sparkat the commutator-brush. If now we prolong the time of commutation, sothat it will end at 3, the line-current will have dropped to zeronamely,to i-and the curthrough the observed coil immediately after commutationwill also begin at zero but the current e, flowing through the coil atthe time of breaking, willbe positive and at k, and hence a spark willresult; but it will not be as large as when the commutation ended at 2.Suppose now that the period of commutation be prolonged to end at 4. Theline-currentwill now be negative and at a minimumnamely, at mand thecurrent which must flow through the observed armature-coil immediatelyafter commutation will be e ual thereto and in the o posite directiontiat is, it will be at n. Th e, current e, flowing through the coil at thetime of breaking the short circuit, is also positive and is likewise atn. Hence the current flowing through the coil at the moment of breakingand that whichanust flow through the coil immediately thereafter are ofequal value and .in the same direction. quently no spark willresult.

In motors as at present constructed and operated the time of commutationis considerably less than one-fourth of the period of the current andmay fairly be represented by the line 2. This, for the reasonsabovestated, causes large sparks; but by continu ing the period ofcommutation for approximatellyhalf of the period of the line-current yto the point 4-no spark will result. In actual practice the currente,'flowing through the armature-coil during commutation, and the currentI), which must flow through the coil immediately after COIlf1m11-tation, may not be exactl equal at the point 4. Nevertheless they wi 1be approximately equal, so that the amount of sparking 'is'negligible.To get the best results, the short circuit of the'coil must be brokenwhen the current flowing in the coil has most closely approached thecurrent which must flow t rough the coil after commutation. vThis, asillustrated, is after a half-period'of the linecurrent. This time,however, will varyshghtly, depending upon the electrical charthearmature-coils-such. as

Consecurrent flowing through their resistance, reactance, capacity, &c.;but with a properly-designed motor it will be approximately after ahalf-period of the linecurrent.

The period of commutation will of course depend upon the speed ofrotation of the armature. The motor will therefore be designed tomaintain the short circuit during a half-period of a current of suchfrequency as the motor is designed for and while running at its averagespeed or speed of greatest'efiiciency. g I

The desired length of commutation may be obtained by making thecommutator-brushes of, sufficient width to bridge adjacentcommutator-bars for the desired period, or by changing the lengths orshapes of the commutator-bars themselves, or in any other suitable way.

While the diagram in Fig. 2 illustrates the commutation as beginning atthe maximum of the line-current and ending at-the 'minimum thereof, thisis done merely for simplicity oi. illustration and ex lana'tion. Thesame results will follow at w atever point in the line-current the shortcircuit'be ins, it only being necessary to continue sai short circuitduring a half-period of the current.

While in the claims it is stated that the per1od.of commutation iscontinued during a half-period of the line-current 'or that it ismaintained until the line-current is equal to the current flowin throughthe coil, it will be understood that t ese phrases are only approximateand that they include all arran ements wherein the commutation ends at te time when the line-current most closely approaches in strength the"current flowing through the coil, even thou h these two cur-. rentsshould not be exacty equal or this should not occur exactly at thehalf-period of the line-current.

What I claim is 1. The method ofcommutating alternating-current motorswhich consists in maintaining the short circuit bf; the arm aturecoilsduring a half-period of the line-current. I

. 2. Thefmethod of commutating alternating-current motors which consistsin maintaining the short circuit of a given armaturecoil until the...commutated line-current is equal to, and in the same direction as,thesaid armature-coil; and 'at'that moment breaking the short circuit;

"Intestimonywhere0f I aflix my signature in presence of two. witnesses.

HANS WEICH'SEL.

Witnesses:

W. F..SHROYER, J. M. NEBBIT.

IIO

